1. Field of the Invention
The present invention relates to methods and apparatus for effecting wafer-level burn-in, or stress testing, of semiconductor devices and, more particularly, to apparatus and methods for establishing an electrical connection between semiconductor devices on a wafer or other substrate including multiple semiconductor devices thereon and burn-in test equipment. Specifically, the present invention relates to apparatus and methods that employ a magnetic field to establish an electrical connection between semiconductor devices carried upon a wafer or other substrate and burn-in test equipment.
2. Background of the Related Art
Once semiconductor devices have been fabricated, the semiconductor devices or representative samples thereof are typically subjected to a series of tests. These tests are intended to determine whether the semiconductor devices will meet various performance and reliability standards.
Stress testing, or burn-in testing, is one of the various types of tests that may be performed on semiconductor devices. Stress testing typically involves the application of a substantial amount of current to one or more semiconductor devices over a prolonged period of time and at an increased temperature or with varied temperature. For example, about 10 milliamps (mA) of current may be applied to each semiconductor device carried upon a substrate as the temperature of the semiconductor device is cycled between ambient temperature and a temperature of at least about 100° C. Such cycling of the temperature of the semiconductor device as current is being applied thereto is intended to stress the semiconductor device by driving any contaminants therein into the active circuitry thereof, thus causing failure of the semiconductor device. This type of stress testing is known in the art to cause the early failure of unreliable semiconductor devices, thereby preventing these unreliable semiconductor devices from being sold and used. As a result, semiconductor devices that pass such stress testing are typically more reliable than those which fail such stress tests.
Conventionally, stress testing equipment has included a carrier configured to hold one or more semiconductor devices during testing and a burn-in oven within which stress testing is conducted. Various types of carriers have been developed, including carriers for single, bare or packaged semiconductor devices, as well as wafer carriers. The carriers may include electrically conductive structures, such as pins, that contact the bond pads of each semiconductor device held by the carrier so as to apply an electrical current or a voltage to each semiconductor device held by the carrier.
Wafer carriers may alternatively be configured to establish an electrical connection with a multiplicity of semiconductor devices carried upon a wafer or other substrate by contacting one or more common contact locations formed on the wafer or other substrate. For example, it is known in the art to fabricate wafers with each of the semiconductor devices carried thereon in communication with a common ground contact and a common power (VCC) contact, which are also carried upon the wafer. Conventionally, electrical connection of the common ground contact and the common power (VCC) contact of such a wafer to ground and a power (VCC) source, respectively, has been effected by use of clamping mechanisms, such as C-clamps or so-called “alligator clips” with planar conductive plates thereon.
When alligator clips are used to establish an electrical connection between the semiconductor devices on a wafer and a ground or a power source, a radial tangent force is applied to the substantially flat active surface and backside of the wafer at the locations of the ground contact and the power (VCC) contact. While a low resistance electrical contact is established by use of such alligator clips, the radial tangent force applied by an alligator clip may cause the conductive plates on the alligator clip to contact only a small area of the respective contact formed on the active surface of the wafer and the opposing backside of the wafer. As a result, a large amount of pressure may be applied to a small area on the wafer, which may cause damage to the wafer that may, in turn, damage semiconductor devices carried by the wafer. In addition, as the temperature of the burn-in oven is increased, the alligator clips may expand and, thus, be moved along the wafer, which may also damage the wafer, as well as the semiconductor devices formed thereon.
While C-clamps contact larger areas of the respective common ground and power (VCC) contacts formed on the active surface of a wafer, as well as larger areas on the backside of the wafer, and apply force to the wafer in a direction substantially normal, or perpendicular, to the plane of the wafer, C-clamps are relatively clumsy and would, therefore, likely increase the chance that a wafer is damaged as C-clamps are secured to their respective contacts. Moreover, when stress testing involves varied temperatures, the expansion of a C-clamp would increase the amount of force applied to the wafer, which could crack or otherwise damage the wafer, as well as semiconductor devices carried upon the wafer. Conversely, contraction of a C-clamp during cooling could result in an inadequate electrical connection between the C-clamp and its corresponding contact.
Due to the material expansion that typically occurs with the temperature variations of burn-in testing, the direction at which contact force is applied to a substrate by both alligator clips and C-clamps may deviate from normal (i.e., from a direction that is perpendicular to the plane of the substrate). As is known in the art, deviations in contact force may cause similar deviations in contact resistance. Even a small change in contact resistance may translate into a substantial drop in the voltage supplied (VCC) to each semiconductor die on the substrate. For example, when the substrate is a wafer that carries 500 semiconductor dice, about 5 amps (A) of current are applied to the power supply (VCC) contact of the substrate, or about 10 mA is supplied to each of the 500 dice. As calculated in accordance with Ohm's law, a small, 20 milliohms (mΩ) increase in the contact resistance between an electrical connector of the burn-in test equipment and a common contact on the substrate would cause a substantial, 100 mV decrease in the voltage (VCC) applied to the dice through the common contact. Thus, the amount of power and voltage applied to each die during wafer-level burn-in testing may not be consistent or repeatable when alligator clips or C-clamps are used to supply a burn-in voltage to dice through a common contact on the wafer or other substrate.
No known apparatus or method for establishing an electrical contact with a common contact on a wafer is available which does not induce stress on or in the wafer. No known apparatus or method in wafer-scale stress testing of semiconductor devices is available without applying too much force or too little force to the wafer.